The trouble with being a kingmaker is that you can't choose the people that have a shot at becoming king. The lone contrarian party isn't in a position to dictate terms; all it can do is decide whether it wants the country to be center-left or center-right on a given issue, which sounds okay for preventing partisan insanity but bad for coherence on anyone's part.

I can only see compromise being a winning move if one of the mainstream coalitions wants to do something that won't work without agreement between several different policy domains, and if it's willing to sacrifice a lot to get it. Otherwise there's no incentive: pissing off the crazies isn't a good strategic move if it implies concessions to your real enemies.